Thermal spray technologies for applying material to surfaces are very well known in the art. Thermal spray coatings can be made from feedstocks of a variety of forms, such as, particulate, suspensions and liquid precursors. When particulate feedstocks are used, typically the particles have diameters varying from 5 to 100 μm. This powder is fed into a thermal spray torch, which has a source of heat. This source of heat can be generated by the combustion of a fuel gas (e.g., acetylene and oxygen) or a plasma gas (e.g., Ar/H2 plasma). The powder particles that tend to melt in the heat source (spray jet) of the thermal spray torch are accelerated (via gas expansion) towards the substrate surface. The molten particles arrive at the substrate surface, where they flatten, cool and solidify forming lamellas or splats. The typical thermal spray microstructure resembles a stack of overlapping splats.
Ceramic materials are known for being hard and stiff. Ceramic thermal spray coatings have been used for many years as anti-wear coatings. Recently, it has been observed that nanostructured ceramic oxide thermal spray coatings exhibit higher wear resistance when compared to their conventional counterparts.
A paper previously published by the applicant (R. S. Lima, A. Kucuk, C. C. Berndt, “Bimodal Distribution of Mechanical Properties on Plasma Sprayed Nanostructured Partially Stabilized Zirconia”, Materials Science & Engineering A, 327, 2002, p. 224-232) teaches that porous non-molten nanostructured particles can be embedded in coating microstructure when thermal spraying with nanostructured partially stabilized zirconia.
There exists a need for a cost effective, simple method of producing porous ceramic thermal spray coatings for a number of applications; for example, to produce abradable coatings for seals, and thermal barrier coatings (TBCs).
Abradable Coatings
Abradable coatings or seals are used in compressors and combustion chambers of aircraft and land-based gas turbines to decrease clearance between e.g. a stator casing and a rotor blade tip, and hence to increase compressor and combustion chamber efficiency, and decrease fuel consumption. Modem turbines require very small clearances between rotating components (blade tips, labyrinth seals) and the stator case in order to minimize gap losses, and increase efficiency. For this purpose, different types of abradable coatings (seals) are deposited via thermal spray on the stator case to cope with rotor misalignment, thermal and centrifugal dilations, and unbalanced parts. The primary requirement of abradable coatings is to allow the coating to wear away without damaging the blade tip.
Abradable coatings are characterized by a friable structure of carefully selected materials. These coatings are difficult to engineer because they must be at the same time readily abradable and mechanically stable to withstand the harsh operating conditions of a gas turbine. There is a demand from the aerospace and energy industries for the production of turbines that operate at higher temperatures, i.e. temperatures higher than about 1100° C. Operation at higher temperatures translates into higher efficiency, higher economy and less pollution. As a consequence, it is desirable that the abradable coatings also follow this trend, i.e., they are able to operate at higher temperatures.
In order to achieve this goal, two main types of high temperature abradable coatings are currently in use. The first one is based on the combination of a high temperature alloy (CoNiCrAlY), a self-lubricating material (BN) and a polymer (polyester). The metallic alloy provides the oxidation resistance and mechanical integrity at high temperatures. The BN lowers the friction coefficient of the coating and the polyester produces high amounts of porosity (producing a friable structure) after it is burned out of the coating.
The second type of high temperature abradable currently in use is based on a ceramic material (ZrO2-6-8 wt % Y2O3), BN and polyester. The ceramic material provides the mechanical and chemical integrity at high temperatures. Like the metallic abradable, the BN also lowers the friction coefficient and the polyester also creates a network of porosity in the coating microstructure (after being burned out), therefore making a friable ceramic material.
Despite the success of the current approaches, there are still problems to be solved. For example, when spraying a composite material with very different physical properties, such as CoNiCrAlY and polyester or ZrO2-7 wt % Y2O3 and polyester, it is very difficult to have consistency in the spraying process, therefore these types of coatings may exhibit homogeneity problems. Further, after coating deposition, the polymer must be burned out of the coating to create porosity. This process takes hours and raises the cost of the process in terms of time and money.
Thermal Barrier Coatings (TBCs)
TBCs are deposited on the surface of metal parts that are routinely subjected to thermal shock (e.g., turbine blades and combustion chambers of aircraft and land based gas turbines, etc.) to decrease heat transfer between e.g. hot gases arising from the combustion of fuel (e.g., kerosene) and the metallic parts. TBCs are normally made of two layers of coatings. The first layer is generally a metallic bond coat (BC), which is deposited directly (via thermal spray) on the metallic surface of the blades and combustion chambers. The BC layer (coating) is usually made of CoNiCrAlY alloys and the typical BC thickness varies from 100 to 200 μm. The main function of the BC is to protect the metallic parts of the turbine against high temperature oxidation and to serve as a support coating or anchor coating for the second layer. The second layer (also known as top coat) deposited (via thermal spray) on the BC layer, is a ceramic coating usually based on zirconia (ZrO2). The typical thickness of the ceramic top coat varies from 250 to 500 μm. The main function of the ceramic top coat, due to its inherent mechanical integrity, stability, low thermal diffusivity/conductivity and chemical resistance up to high temperatures, is to protect the metallic parts of the turbine against the high temperature environment of the combustion of fuel in the turbine engine. With the use of TBCs it is possible to increase the compressor and combustion chamber efficiencies (by burning fuel at higher temperatures) and decrease fuel consumption. Today, most of the aviation and land based gas turbines make use of TBCs.
There is a demand from the aerospace and energy industries for the production of turbines that operate at higher temperatures, i.e., temperatures higher than 1100° C. Operation at such higher temperatures would translate into higher efficiency, high economy and less pollution. New materials such as La2Zr2O7, SrZrO3 and BaZrO that are more stable at higher temperatures and present a low thermal conductivity have been proposed recently to address this need but their fracture toughness is lower than zirconia-based TBCs making them more prone to delamination near the BC interface.
In order to provide higher combustion temperatures, it is important to engineer TBCs with lower thermal diffusivity, thermal conductivity and elastic modulus values, when compared to those of the current TBCs. A low elastic modulus of the ceramic topcoat makes it possible to reduce the thermal stresses at the top coat/BC interface arising from the difference in thermal expansion coefficients between the two layers. As the BC is not responsible for the thermal protection performance of the TBC, the ceramic top coat will have to be re-engineered or further developed in order to produce a structure which will lower the thermal transfer from the combustion gases to the metallic parts of the turbine. Moreover, there is always a demand for more reliable TBCs that will last longer and confer a better thermal protection of the metallic substrate in gas turbines as well as in diesel engines, internal combustion engines, and in general any metal surface that is coated for thermal protection.
It has been very, widely established that zirconia-based coatings are not suitable for use as a TBC, principally because zirconia-based coatings are known to sinter and densify in high temperature environments. The smaller the crystal size the faster they sinter. For these reasons the zirconia-based coatings would be expected to become very rigid and would be expected to crack under thermal shock conditions.
It will thus be appreciated that thermal spray coatings have important applications and that a wide variety of thermal, mechanical and chemical properties of coatings may be desired.